Transient simulation of nitrogen oxide emissions on diesel engines

Zusammenfassung

In recent years the laws concerning engine emissions have become ever stricter. For the development of new engines it is therefore more important than ever to achieve predictions of the emissions via simulation which are as accurate as possible. Due to stricter legislation with lower emission limits and more transient driving cycles or even Real Driving Emissions, transient emissions are becoming a relevant aspect of this need for accurate predictions via simulation. This report presents a guideline for transient simulation of nitric oxide emissions on diesel engines.

Simulation of transient nitric oxide emissions requires more than just a transient capable emissions model. For true transient simulations of the emissions the whole process chain has to be transient capable. This typically starts with a transient capable flow model which provides not only the gas dynamics but, as a very important input factor, the response characteristics of the turbo charger as well. The next very important step in the process chain is the burn rate model. Only a transient capable model can predict the burn rate for all of the continuous intermediate steps of dynamic operations correctly. In this paper a quasi-dimensional model, which calculates the burn rate based on the injection rate, was used. Accordingly, the injection rate has to be simulated by a transient capable injection model to deliver a usable basis for the burn rate model.

This transient capable process chain delivers the necessary boundary conditions for the emissions model to simulate the transient nitric oxide emissions. Since the preceding models are transient capable many transient influences on the nitric oxide emissions, such as transient boost pressure, EGR rate, cylinder charge, charge air temperature, rail pressure etc., are already allowed for. However, during analyses of measured transient nitric oxide emissions further direct influences have been found, which have to be integrated directly into the emissions model.

A direct influence of the combustion chamber wall temperatures on the nitric oxide emissions can be shown based on measured load steps, as well as additional stationary measurements. Load steps to and from, as well as stationary measurements at low global combustion air ratios were used to examine the behavior of nitric oxide formation under these operating conditions. These two principal points are particularly important for the simulation of transient emissions but also contribute to improving forecasting at stationary operating points and to enabling forecasting to occur at all in the case of low global combustion air ratios.

With all of those factors considered the simulation of transient emissions on diesel engines witch adequate precision becomes possible.